Nearly all modern electrochemical energy conversion devices, including fuel cells, batteries, and electrolyzers, utilize high surface area graphitic carbon to pass charge to the active components of the device. The high surface area carbon serves to maintain a catalyst dispersion, to provide a mesoporous network suited to efficient mass transport, and to provide low resistance to charge transfer to the active phase of the device. But under the harsh electrochemical conditions typical of these devices, this carbon support is susceptible to corrosion, a principal cause of premature device failure. Moreover, surface oxides formed during degradation of the carbon support catalyze the generation of hydrogen gas, compromising device safety and eroding system efficiency.
Carbon, in all its allotropes, is thermodynamically unstable with respect to oxidation to CO2. Indeed, the thermodynamic potential for the oxidation of graphitic carbon to CO2,C+2H2O→CO2+4H++4e−  Equation (1)is 0.207 V vs a normalized hydrogen electrode (NHE)—nearly a volt lower than the potentials typical of polymer electrolyte membrane (PEM) fuel cells and high potential lithium ion and lithium air batteries. The following generic corrosion sequence is the likely consequence:
in which Cs denotes a surface carbon atom. The first step in the sequence, the oxidation of a carbon-hydrogen bond, is presumed to be rate limiting. Under the oxidizing environments found in fuel cell, lithium ion, and lithium air battery cathodes, the oxidation events proceed via reactive oxygen species such as superoxide or high potential metal oxides. As such, the elementary surface reaction that initiates the oxidation cascade is a hydrogen atom abstraction in which carbon-hydrogen (C—H) bonds react with oxygen-centered radicals to generate oxygen-hydrogen (O—H) bonds, leaving behind a carbon-based radical that is subsequently converted into a C—O bond.
Moreover, carbon-based electrodes are able to catalyze the production of hydrogen gas. Hydrogen evolution catalysis proceeds via surface-bound hydrogen atoms, and surface oxides on carbon electrodes will bind hydrogen atoms relatively readily: at pH 7, onset of hydrogen evolution occurs at an approximate overpotential (η) of 0.75 V. At these potentials the fraction of charge that goes toward hydrogen production instead of the desired battery charge/discharge reaction can be considerable. This poses a substantial problem for all aqueous redox flow batteries because parasitic hydrogen production causes an imbalance in the population of the active redox species in each half cell. Over several hundred cycles, this imbalance adds up, leading to a steady decay in storage efficiency and premature device failure. To mitigate this problem, commercial redox flow batteries must utilize an auxiliary fuel cell to oxidize the parasitic hydrogen back to water and return the charge to the cell to rebalance the electrolyte populations.
Attempts have been made to passivate carbon electrodes by treatment with fluorine gas at high temperatures, with fluorine plasmas, and by mixing fluoropolymers with carbon substrates. But such techniques are variously time-consuming, energy-intensive, require high vacuum, and compromise conductivity through the carbon network.